1. Field of the Invention
The present invention related to a data demodulator, a data demodulation method and a program thereof, and particularly relates to a data demodulator, a data demodulation method and a program capable of allowing recording/reproducing characteristics to be more stable.
2. Description of the Related Art
When data is transmitted to a given transmission line or recorded in recording media, for example, a magnetic disc, an optical disc, a magneto-optic disc and so on, data is modulated so as to be suitable for the transmission line or recording media. As one of such modulation methods, block coding is known. In the block coding, a data row is divided into blocks in a unit of “mxi” bits (referred to as a data word in the following description) and the data word is converted into a code word of “nxi” bits in accordance with a suitable code law. The bit of the code word is also referred to as a channel bit in the following description. In this coding, the code will be a fixed length code when i=1, and the code will be a variable length code when plural “i”s can be selected, that is, when a given “i” is selected from a range of 1 to “i max” (maximum i) to perform conversion. The code to which the block coding has been performed is represented by a variable length code (d, k; m, n;r).
Here, “i” is referred to as a constraint length and “i max” is represented by “r” (maximum constraint length). Additionally, “d” represents, for example, the minimum number of continuing “0”s entering between continuing “1”s, namely, the minimum run of “0”, and “k” represents the maximum number of continuing “0”s entering between continuing “1”s, namely, the maximum run of “0”.
In the case of recording the cord words obtained as described above in the optical disc, the magneto-optic disc and so on, an NRZI (NonReturn to Zero Inverted) modulation in which the waveform is inverted at “1” and is not inverted at “0” is performed by a variable-length code row to perform recording based on the variable length code to which the NRZI modulation has been performed (referred to as a recording waveform row in the following description), for example, in a compact disc (CD) and a minidisc (MD) (Trademark). This is called mark edge recording. On the other hand, in the magneto-optic disc of 3.5 inch/230 MB capacity in an ISO standard and the like, a code row to which recoding modulation has been performed is directly recorded without NRZI modulation. This is called mark position recording. The mark edge recording is often used in recording media having high recording density at present.
When the minimum inversion interval of the recording waveform row is Tmin and the maximum inversion interval is Tmax, it is preferable that the minimum inversion interval Tmin is longer, namely, the minimum run “d” is higher for performing the high-density recording in the linear velocity direction and it is preferable that the maximum inversion interval Tmax is shorter, namely, the maximum run “k” is lower for performing reproduction of a clock. It is also preferable that Tmax/Tmin are lower when considering overwrite characteristics. Various modulation methods have been further proposed and practiced while checking conditions of media such that it is important that the detection window width Tw=m/n is wide in terms of Jitter or S/N.
Here, modulation methods which have been proposed or practically used in the optical disc, the magnetic disc, or the magneto-optic disc and so on will be specifically cited. EFM code (also written as (2,10;8,17;1)) used in CD or MD and 8-16 code (also written as (2,10;1,2;1)) used in DVD (Digital Versatile Disc) and RLL(2,7) (also written as (2,7;m,n;r) used in PD (120 mm 650 MB capacity) are RLL codes in which the minimum run d=2. Additionally, RLL (1, 7) (also written as (1,7;2,3;r)) used in MD-DATA2 or 3.5 inchMO (640 MB capacity) in an ISO standard is an RLL code in which the minimum run d=1. Furthermore, the RLL code (Run Length Limited code) having the minimum run of d=1 is often used, in which the size of the minimum mark and conversion efficiency are well balanced in a recording/reproducing disc apparatus for the optical disc, the magneto-optic disc and the like having high recording density which is currently under development and study.
A modulation table of the variable length RLL (1,7) code is a table, for example, shown as follows:
TABLE 1RLL(1, 7): (d, k; m, n; r) = (1, 7; 2, 3; 2)data patterncode patterni = 11100x100100110xi = 20011000 00x0010000 0100001100 00x0000100 010
A sign “x” in the modulation table indicates “1” when a subsequent channel bit is “0”, and indicates “0” when a subsequent channel bit is “1”. The maximum constraint length “r” is 2.
Parameters of the variable length RLL (1,7) are (1,7;2,3,2), and when a bit interval of the recording waveform row is T, the minimum inversion interval Tmin represented by (d+1)T is 2(=1+1)T. When the bit interval of the data row is Tdata, the minimum inversion interval Tmin represented by (m/n)×2 will be 1.33(=(⅔)×2)Tdata. The maximum inversion interval Tmax represented by (k+1)T will be Tmax=8(=7+1)T(=(m/n)×8Tdata=(⅔)×8Tdata=5.33Tdata). Furthermore, the detection window width Tw is represented by (m/n)×Tdata, and a value thereof will be Tw=0.67(=⅔)Tdata.
In a channel bit row to which the modulation by the RLL (1,7) of Table 1 has been performed, generation frequency of 2T is the most frequent which is Tmin, and the order of frequency will be 3T, 4T, 5T, 6T . . . . A case in which 2T as the minimum run is repeated, that is, the case in which a lot of edge information is generated in early cycle is advantageous in clock reproduction in many cases.
However, when the recording linear density is further increased in recording/reproducing of, for example, an optical disc, the minimum run will be at a portion where an error is liable to occur. Because a waveform output of the minimum run is lower than other runs at the time of reproducing a disc and is liable to be affected by, for example, defocus, tangential tilt and the like. Furthermore, recording/reproducing of continuing minimum marks in the high recording linear density is liable to be affected by disturbance such as noise, therefore, reproduction errors of data are liable to occur. As a pattern of the reproduction errors of data at this time, a case in which all continuing minimum marks from the head edge to the last edge are shifted and fail all at once. That is, the bit error length to be generated is propagated from the head to the last in a segment in which the minimum runs continue. Accordingly, there arises a problem that the error propagation length will be longer.
For stabilization at the time of recording/reproducing data with high linear density, it is effective to set limits on continuity of the minimum runs.
On the other hand, at the time of recording data in a recording medium or transferring data, coding modulation suitable for the recording medium or a transmission line is performed. When low frequency components are included in these modulated codes, variation is liable to occur in various types of error signals such as a tracking error in servo control of a disc apparatus or jitter is liable to occur. Therefore, it is desirable that low frequency components are suppressed in modulation codes.
As a method of suppressing low frequency components, there is DSV (Digital Sum Value) control. DSV means the total sum calculated by NRZI modulating the channel bit row (namely, level coding) to obtain a recording code row and allowing “1” in the bit row (symbol of data) to be “+1” and allowing “0” to be “−1”. The DSV will be the measure of low frequency components of the recording code row. To decrease absolute values of variation of positive and negative in the DSV, namely, to perform the DSV control leads to removal of DC components of the recording code row and suppression of low frequency components.
The DSV control is not performed in the modulation codes by the variable length RLL (1,7) table shown in the Table 1. The DSV control in this case is realized by performing the DSV calculation at given intervals in the coding row (channel bit row) after modulation and given DSV bits are inserted into the coding row (channel bit row) (for example, refer to JP-A-11-177431 (Patent Document 1)).
The number of DSV bits inverted into the channel bit row is determined by the minimum run “d”. When the DSV bit is inserted in an arbitrary position in the code word so as to secure the minimum run under the condition that d=1, the necessary number of bits is 2(=d+1)-channel bit. The necessary number of bits when the DSV bit is inserted in an arbitrary position in the code word so as to secure the maximum run is 4(=2×(d+1))-channel bit. When the DSV control is performed with the number of channel bits lower than the above, it may be difficult to perform the DSV control depending on adjacent patterns between which the DSV bit is sandwiched.
In the RLL (1,7) code in which (d,k;m,n)=(1,7;2,3), when the DSV bit is converted into data with the conversion rate:
4-channel bit×⅔=8/3=equivalent of 2.67 data (2.67 Tdata).
The DSV bit is a basically a redundant bit. Therefore, it is preferable that the number of DVS bits is reduced as small as possible from the viewpoint of efficiency of coding conversion.
Furthermore, it is preferable that the minimum run “d” and the maximum run “k” do not vary according to the DSV bit to be inserted. This is because variation of (d,k) affects the recording/reproducing characteristics.
In the actual RLL code, it is necessary that the minimum run is inevitably secured as it has a significant impact on recording/reproducing characteristics, however, the maximum run is not always secured. There exists a format using a pattern breaking the maximum run as a synchronization pattern in some cases. For example, the maximum run in 8-16 code of the DVD (Digital Versatile Disc) is 11T, however, 14T which exceeds the maximum run is given in a synchronization pattern portion to thereby increase the detection performance of the synchronization pattern.